"The boys drink the water."
Translation:De jongens drinken het water.
You will just have to memorize which nouns use "het" or "de" for "the." There are no clear rules, except for the ones on the bottom of the page on the "Basics 1" lesson.
How do you tell is something is gendered or not? Obviously girls and men and such are gendered, but why do milk and newspaper have genders and water doesn't? My issue is not with the language, but if Duolingo isn't going to provide us with the gender of a word, they can't expect us to know when to use de and het.
Everything has a gender. The words themselves have the gender, not the actual object. For this reason "girl" is a neuter noun even though it is a female. Many languages do this. I see you are learning Spanish, so think of it as "el libro" is masculine, not because the book is masc., but because the word itself is (it ends in -o).
Dutch has three genders, masculine, feminine, and neuter. But because masculine and feminine nouns act the same, there is no need for distinction at the basic level. As with some other germanic languages, the gender is not easily recognized as it is with other languages like Romance ones. You have to memorize the gender of each noun. There are tips to knowing some gender, (Girl is neuter, because it is a declination, it ends with -je).
I wish Duolingo had the gender attached to all the nouns, but they don't. The Dutch team posted somewhere that they decided against it. If you wish to learn the gender of the nouns before working with them, you could try... http://www.memrise.com/course/356226/duolingo-dutch-for-english-speakers-3/